BREAKDOWN MECHANISM OF LIQUID-NITROGEN VIEWED FROM AREA AND VOLUME EFFECTS

We investigated the area and volume effects on the breakdown strength in liquid nitrogen (LN2) to discuss breakdown mechanism in cryogenic l iquids for superconducting power apparatus. We measured breakdown volt ages in LN2 with and without thermal bubbles over a very wide range of the electrode size. Experimental results revealed that breakdown mech anism changed from area dominant to volume effective region at the lar ger electrode configurations in LN2. Moreover, we discussed the contri bution rate of area and volume effects to the breakdown strength in LN 2. It was suggested that mutual contribution of area and volume effect s appeared in breakdown characteristics in LN2 under thermal bubble co ndition, as a phenomenon peculiar to cryogenic liquids. Consequently, we pointed out that it is very important to consider both thermal bubb les and electrode surface condition for HV insulation of superconducti ng power apparatus.